Do starter capacitors ever fail gradually, or do they either work or not work?

I have a rooftop evaporative aircon, which is playing up. The motor starts, but
accompanied by loud humming and the fresh smell of burning electrical
insulation.
Starter capacitor in the control box (it is a variable speed controller) is
nominated as 25uF (+- 5%), and reads with a multimeter as 22.74uF. No bulges or
deformities on the starter cap.

Should this reading be exactly as printed on the cap?

I have checked all the wiring into the control box, and out to the various
accessories (fan motor, water pump motor & downstairs control box -with the
user-selectable switches in it for the aircon), and there dont seem to be any
problems there.

Water pump starts happily, with no humming, and no burning electrical smell.

My suspicion is I have a fan motor about to expire. The electrical smell doesnt
manifest itself in the rooftop control box, and only appears when the fan is
started.

Info on the death of starter capacitors appreciated.

-----------------

Cheers,

Rod.....Out Back

==================================================
For a selection of my pics from the past 12 months,
visit the web site below:
http://www.flickr.com/photos/rod_outback /

As Phil said, it's weird that this kind of cap would fail with high ESR.
BTW and just for everyone's info, DSE are flogging all their kits at
ridiculously low prices at the moment. The ESR meter is just under $25
when at one time it was up near $80.

its probably a failure of the end-cap metallisation. If I had access to
ABSE I could post a photo of a partially assembled film cap - I have one
here, its 80mm OD, 100mm long and measures 109uF on my Fluke 12.

If the metallisation process is poorly controlled, the
metallisation-foil interfaces wont be very good (high resistance) and
thumping great wads of current can blow them open. Its a runaway kinda
thing too....

wow, I'll see if I can get one here in NZ for that sort of price! I cant
find it on the NZ website though...

I dunno what's going on with the Kiwi DSE website. I can't find the
ESR meter by catalog number. All I managed to find by searching for
'ESR' was the chart for the first version's front panel.
I couldn't find the K7205 LOPT tester either. Looks like DSE's not
stocking a lot of their kits over there now.
Does anyone know anything about this?

it should do; 30 minutes will suffice. at present I use a signal
generator, a little PCB I made and a small amount of arithmetic. But at
that price, its silly not to get one.

interestingly enough, I want it to measure motor run caps. I'm finishing
off a design now that starts and runs three-phase SCIMs on a single
phase supply, with no inrush (which is a fairly good trick). Suffice it
to say I use +24Vdc and -900Vdc to achieve this, along with some baby
1200V 30A SCRs.

oh yeah. I've got a bus discharge resistor I made up in the 90's when I
was a design engineer at PDL electronics, designing motor controllers.
Its a pair of 50R resistors in series, each rated for 11kW peak pulse
power, coated in heatshrink and attached to a pair of fluke DMM leads.
the tips are a lot shorter now than when I started ;)

We used these in our production test area to discharge the DC bus of
drives that had been tested. a 400V 660A drive had 19.8mF at 800V (after
regeneration) so about 6.4kJ peak; from the AC line it was a mere 560V,
so only 3.1kJ. The 1MW drives were three 660As in parallel, so almost
20kJ in the DC bus.

occassionally the test tech would forget to turn the drive off before
discharging the bus. big problem, as you cant remove the leads, it just
arcs, shorts the DC bus and BANG goes $50,000 worth of product. And if
you left it there, the resistor went bang (and it typically pointed at
ones groin). So they'd scream, and someone would hit an E-stop and knock
out all the test bay power.

they scared the shit out of me during my interview. I'd never seen 2"
thick wires before, let alone ones that jumped about, and made watches
spin crazily. rubber mats and one hand in the pocket was de rigeur, nad
nobody worked alone. But their most serious accident was an office lady
falling off a mezzanine floor, and braining herself on the concrete floor.

although one night the fire alarm went off, so me and a couple of other
engineers went to investigate (naughty us). a 600A test machine had
poled, and flames were pouring out of it. we hit an E-stop, and it didnt
work. So we pulled a trip cord, and that didnt work either. I ended up
leaning over the drive, scrolling down to the start/stop screen and
telling the drive to stop.

Turned out that morning marketing had been taking photos in the test
area, and made the guys take the trip cords down. but they were
fail-safe, so someone had jumpered them with clip leads. which
conveniently disabled all the E-stop buttons too. Only problem was they
forgot to remove the clip leads.....

There might be other really cheap kits you'd have more use for? The
ESR meter is awfully specialised.
If you decide to get one, be aware that DSE's selling the Mk1 (1996)
version of the meter for the same price, and the Mk1 and Mk2 cartons
tend to get mixed up on the shelf.

Thanks Bob. I will go and have a look today. Its one of those tools
that occasionally you say 'I wish I had one of those'. Whilst i very
rarely have anything to do with hardware it does still arise from time
to time.